1. Field of Invention
Embodiments of the present disclosure pertain to resin infusion and polymer processing for composite manufacturing and, in particular, to a combination of an epoxy curing agent and an engineering thermoplastic.
2. Description of the Related Art
Fiber-reinforced polymer matrix composites (PMCs) are high-performance structural materials that are commonly used in applications requiring resistance to aggressive environments, high strength, and/or low weight. Examples of such applications include aircraft components (e.g., tails, wings, fuselages, and propellers), high performance automobiles, boat hulls, and bicycle frames. PMCs comprise layers of structural reinforcement fibers that are bonded together with a matrix material, such as a polymer resin. The structural reinforcement fibers reinforce the matrix, bearing the majority of the load supported by the composite, while the matrix bears a minority portion of the load supported by the composite and also transfers load from broken fibers to intact fibers. In this manner, PMCs may support greater loads than either the matrix or fiber may support alone. Furthermore, by tailoring the structural reinforcement fibers in a particular geometry or orientation, the composite can be efficiently designed to minimize weight and volume while maximizing strength and performance.
Numerous processes have been developed for the manufacture of PMCs. These include Liquid Molding (LM) and preimpregnated prepregs (prepregs).
Conventional Prepregs incorporate sheets of structural reinforcement fibers that are wetted, impregnated, with a matrix resin. These prepregs are then layered onto each other in a particular orientation on a tool to form a laminate where they are then subjected to heat and pressure in an autoclave to cure the prepreg layup into the final composite.
The Liquid Molding approach differs from that of conventional prepreg in that dry structural reinforcement fibers are placed into a mold cavity or other mechanism for net-shape tooling and a matrix resin is injected or infused into the structural reinforcement fibers. Liquid Molding (LM) is a generic term which covers processing techniques such as Resin Transfer Molding (RTM), Liquid Resin Infusion (LRI), Resin Infusion under Flexible Tooling (RIFT), Vacuum Assisted Resin Transfer Molding (VARTM), Resin Film Infusion (RFI) and the like. The potential benefits of LM over a conventional prepreg route include reduced lay-up time, a non-dependence on prepreg tack and drape and increased shelf life properties. In practice, the use of LM technology finds its greatest use in specialized operations in which complex composite structures (multi components) are required, locally strengthened structures are required by selectively distributing carbon fibers in the mold and where the need for very large structures is required e.g., marine applications.
Resin Film Infusion (RFI) is a technique that combines an LM technology with conventional prepreg, e.g., in RTM or RFI autoclave curing, where individual prepregs are stacked in a prescribed orientation to form a laminate, the laminate is laid against a smooth metal plate and covered with successive layers of porous Teflon®, bleeder fabric and vacuum bag. A consolidating pressure is applied to the laminate, to consolidate the individual layers and compress bubbles of any volatile that remain.
The use of an autoclave creates a limit to the size of the components that is possible to produce, however. For example, it is not possible to build large structures such as a boat hull, an aircraft wing or fuselage, or a bridge, using an autoclave because that would require an equally large autoclave adding enormous capital costs and running costs.
VARTM simplifies hard mold RTM by employing only one-sided molds, and using vacuum bagging techniques to compress the preform. However mold filling times can be far too long, if indeed the resin does not cure before total fill.
RIFT provides much faster fill times. A distribution media, that is, a porous layer having very low flow resistance, provides the injected resin with a relatively easy flow path. The resin flows quickly through the distribution media, which is placed on the top of the laminate and then flows down through the thickness of the preform. The use of fibers to create channels for the resin infusion is known (WO0102146A1 (Plastech), U.S. Pat. No. 5,484,642 (Brochier), U.S. Pat. No. 5,326,462 (Seemann)) however these channels are either removed during the degassing and curing stage or if they are left in they remain intact post cure.
The matrix resins require various mechanical properties in a final composite including strength and toughness. While most thermosetting polymers result in sufficient strength, they are often brittle and their toughness or resistance to damage is low. As a result, numerous methods have been employed to increase toughness over the years including the incorporation of tough thermoplastics into the matrix resin. For conventional prepreg systems the thermoplastic can be added directly into the matrix resin and then impregnated into the structural reinforcement fibers. However, thermoplastics increase the viscosity of the matrix and increase the difficulty of prepreg manufacturing. In addition, the increased matrix resin viscosity makes LM using thermoplastic toughened matrix resins unmanageable because the high viscosity resin is too difficult to inject into the structural reinforcement fibers.
Additionally, although many thermoplastics are tough, ductile materials, their use in aerospace structural materials has been minimal for several reasons. First, many thermoplastics do not have the solvent resistance, thermal stability, and high softening points required for demanding aerospace applications. Second, the high temperature engineering thermoplastics are difficult to process; often requiring both high temperature and pressure to produce acceptable carbon fiber reinforced composite parts. Therefore, because thermoplastic polymers are subject to high temperature degradation there is a narrow processing temperature window between the processing temperature and the temperature at which the thermoplastic degrades.
Due to the difficulty of incorporating thermoplastics into resins for resin infusion applications, various attempts have been made to separate the beneficial thermoplastic toughening element from the resin. These include the use of thermoplastic veils, fibers and mats integrated within the preform, thus allowing the infused resin to be free, or virtually, free of thermoplastic. These thermoplastic toughening elements incorporated within the preform can be insoluble or soluble when the resin is infused.
When resin soluble thermoplastics are incorporated within the preform, the thermoplastic will only dissolve into the infused resin matrix when the resin system is heated to the dissolution temperature of the soluble fiber. Further, because the dissolution temperature of soluble fiber may be high, the temperature may reach a point that will cause the soluble fiber to degrade.
Another drawback of the liquid molding resin infusion technique is that matrix resin in conventional resin systems must contain the curing agent for the epoxy resin thermosetting system. By requiring the epoxy curing agent in the resin itself, the combination of the resin and epoxy curing agent result in a short pot life of the infusible resin.
Thus, there is a need in the art for a liquid molding resin infusion technique wherein the pot life of the resin is increased, the resin viscosity is lowered for injection and the dissolution temperature of a soluble thermoplastic fiber is lowered in the resin. Also, the soluble fiber processing temperature needs to be lowered, which would broaden the processing window to avoid degradation.